Space‐Confined Donor‐Acceptor Strategy Enables Fast Spin‐Flip of Multiple Resonance Emitters for Suppressing Efficiency Roll‐Off.

Multi‐resonance boron‐nitrogen‐containing thermally activated delayed fluorescence (MR‐TADF) emitters have experienced great success in assembling narrowband organic light‐emitting diodes (OLEDs). However, the slow reverse intersystem crossing rate (kRISC) of MR‐emitters (103–105 s−1) that will lead to severe device efficiency roll‐off has received extensive attention and remains a challenging issue. Herein, we put forward a "space‐confined donor‐acceptor (SCDA)" strategy to accelerate RISC process. The introduction of SCDA units onto the MR‐skeleton induces intermediate triplet states, which leads to a multichannel RISC process and thus increases kRISC. As illustrated examples, efficient MR‐emitters have been developed with a sub‐microsecond delayed lifetime and a high kRISC of 2.13×106 s−1, which enables to assemble high‐performance OLEDs with a maximum external quantum efficiency (EQEmax) as high as 32.5 % and an alleviated efficiency roll‐off (EQE1000: 22.9 %). [ABSTRACT FROM AUTHOR]